Tufts of yarn are installed on a Boeing 757 tail at NASA's Ames Research Center's wing tunnel to visualize the airflow over the tail. The sweeping jet actuators are located roughly in the middle of the vertical tail's surface, just to the right of the rudder. Photo: NASA Ames Research Center

Jason Paur
Gear

11.19.13

3:30 pm

Boeing Adds Tiny Holes to Its Biggest Plane to Boost Efficiency

Image: Boeing

Boeing’s new 777X is getting a lot of attention for its composite wing with folding tips and its super-efficient engines, but one of the airliner’s most innovative features are the tiny holes in its tail that smooth airflow and improve fuel efficiency.

The holes help smooth airflow around the tail by improving something called laminar flow, basically making the airplane more aerodynamic, which reduces fuel consumption. “Aerodynamic advances such as a hybrid laminar flow control vertical tail,” are the few words Boeing used to describe it in a press release announcing the 777X, Boeing’s impressive updating of its long-range twin-engine airliner. Boeing is working with NASA to further develop the idea to include “sweeping jet actuators” embedded in the tail of future models. Such advancements in airflow manipulation could bring significant fuel savings.

Laminar flow occurs when when air flows smoothly over a surface. Think of a water flowing smoothly around a large, smooth round rock. The water flows easily, and the water appears glassy. But if the rock is jagged or the water is flowing fast enough, turbulent eddies form. The same thing happens at the micro scale as an airplane moves through the air, and that turbulent flow increases drag. Increased drag leads to increased fuel consumption.

The challenge is that laminar flow is difficult to maintain over the entire surface of a wing or tail. As the air flows past the leading edge, it becomes more turbulent, separating from the surface and increasing drag.

Photo: Jason Paur/WIRED

Boeing has come up with an innovative solution with its hybrid laminar flow control system, but is keeping most of the details under wraps. The company is using a similar innovation on the 787-9, the stretched version of the Dreamliner, providing a few clues. Tiny holes covering the unpainted leading edge of the 787-9’s vertical tail are used to control the airflow over the surface (the leading edge of the vertical tail on the 777X in the artist rendition at top is also unpainted). Turbulent airflow is reduced through suction as air pulls the turbulent layer through the small holes. This technique has been researched for decades (.pdf), including research by NASA on the F-16XL and more recently by Boeing rival Airbus on an A320 test aircraft in the late 1990s. By ingesting the turbulent layer of air through the tiny holes, the overall drag over the tail surface is reduced.

The next generation of the idea is active flow control that blows small amounts of air out of tiny “sweeping jet actuators” located just ahead of the rudder. NASA and Boeing recently concluded wind tunnel testing of the design and plan to make inflight tests a few years down the road.

The rudder is used to control the airplane’s yaw, or sideways motion. Rudder design is largely dictated by rare situations such as extremely strong crosswinds during landing, and handling the loss of power in one engine during take off. The rudder of an airplane has to be big enough that if an airplane were to lose power in an engine at the moment of take off, the pilots would have enough “rudder authority” to counter the added drag of the engine that is no longer working. For that reason, rudders tend to be oversized for normal flight. But if you can maintain laminar airflow over the rudder, you make the rudder more effective — and therefore you can make it smaller and it can still perform as needed. A smaller rudder creates less drag and weighs less, which increases fuel efficiency.

Tufts of yarn are installed on a Boeing 757 tail at NASA’s Ames Research Center’s wing tunnel to visualize the airflow over the tail. The sweeping jet actuators are located roughly in the middle of the vertical tail’s surface, just to the right of the rudder. Photo: NASA Ames Research Center

Boeing and NASA’s initial testing of the idea ended earlier this month at the agency’s wind tunnel in Moffet Field, California (video below). Engineers installed the vertical tail from a Boeing 757 that had been modified with 37 sweeping jet actuators. The small jet actuators essentially blow air very precisely over the surface of the tail in a sweeping motion. The added airflow from the jet actuators mounted on the tail just upstream of the rudder energize the airflow over the rudder, allowing the air to stay attached over more of the surface of the rudder when it is deflected, which is what increases its effectiveness.

When used during a flight on an actual airplane, the jet actuators would only be turned on when the rudder is deflected past a certain angle, typically only seen during takeoff and landing. This would provide pilots with the added flight control needed during those critical phases of flight.

Boeing plans to install the active flow control jet actuators on a 757 demonstrator aircraft with flight tests planned for 2015. The added laminar flow and reduced tail size needed on a future aircraft design equipped with the technology could lead to fuel savings of 1-2 percent, a significant amount for the airlines, who count every drop.